U.S. Pat. No. 4,087,394 (Concannon) discloses an aqueous dispersion of certain fluoropolymers which contained dissolved film-forming material (binder) for forming a non-stick coating, the weight proportion of fluoropolymer-to-binder being 1.6:1. The dispersion was applied to a substrate such as aluminum sheet to provide a non-stick coating. The coating is disclosed to be a stratified composition, with the binder being concentrated at the substrate surface to provide adhesion of the coating to it and the fluoropolymer being concentrated at the outer surface to provide the release property. For adequate adhesion to the substrate, it became customary to grit-blast or otherwise roughen the substrate surface.
It became desirable to eliminate the step of roughening the substrate surface, requiring that the substrate receive multiple coatings of fluoropolymer, the first coat being the primer layer adhering the remaining layers to the substrate, e.g. the topcoat layer and possibly a midcoat layer between the primer layer and topcoat layer. Adequate adhesion to the smooth surface substrate was achieved by compositions which promoted stratification within the fluoropolymer/binder primer layer. In this case, the concentration of fluoropolymer away from the substrate surface had to provide adhesion to the fluoropolymer mid- or topcoat next applied.
Primer compositions promoting stratification are disclosed in U.S. Pat. Nos. 5,168,013 and 5,240,775 (both by Tannenbaum), the fluoropolymer component of the fluoropolymer/binder composition being the combination of polytetrafluoroethylene (PTFE) of low and high melt viscosity or of PTFE and TFE/perfluoro(alkyl vinyl ether) (PAVE) copolymer, respectively. PCT WO 94/14904 (Thomas) discloses the fluoropolymer component to be two different fluoropolymers, generally having a lower melt viscosity than the Tannenbaum PTFE and the binder component being the combination of two different binders of specified identity. The primers prepared in these references were applied to smooth, degreased aluminum and had fluoropolymer-to-binder ratios of 2.5, 2.4, and 1.5:1, respectively. In Thomas, the composition is applied by roller coating and cannot be applied by spray coating.